Analyzing system for determining the fundamental frequency of a complex wave



Patented July 24, 1951 UNITED STATES PATENT OFFICE ANALYZING SYSTEM FORDETERMINING THE FUNDAMENTAL FREQUENCY F A COMPLEX WAVE Application May28, 1948, Serial No. 29,685

9 Claims.

This'invention relates to wave translation and vsignaling systems, andin its more specific aspects isconcerned with the analysis of complexsignal waves.

The invention has as one of its objects the determination of thefrequency of the fundamental component of complex waves such as, forexample, speech, or other signal waves.

A' further object of the invention is to eiect this determinationregardless of the presence or absence of the fundamental component ofthe wave.

It is also an object of the invention to improve systems for reducingthe frequency range required for the transmission of signals, such asfacilitating the transmission of a message over a transmission mediumnot adapted to accommodate the band with of the message in its originalstate.

' It is known that substantially all of the intelligence contained incertain types of complex signal waves may be transmitted while usingonly a fraction of the frequency space occupied by the wave in itsoriginal state. This is particularly true of speech signal waves; andsignaling systems'employing this principle have been proposed. One suchsystem, as disclosed in United States Patent 2,151,091, March 21, 1939,to H. W. Dudley, analyzes the original signal wave in terms of its fixedand Variable parameters, to secure information concerning both itsfrequency and its amplitude patterns. As there explained, the humanvocal system is considered to be composed of both xed land variableparts, which dilerentiation may be applied to characterize the types ofsignal they produce. So considered, the fixed features correspond to theoscillatory sound produced with the Various parts of the vocal system intheir average or normal condition which includes an average vocal cordtension for the voice sound. The Variable features correspond to thechanging, or modulating 'of the system by varying the different vocalsystem parts from their average condition. In accordance with thissystem, substantially all of the intelligence of a speech signal can betransmitted between two points by transmitting only informationconcerning the variable parameters, since the fixed parameters areequally known at those points.

' Speech signals vary more or less continuously, and relatively slowly,between .two distinct types. In one type, there is a fundamentalfrequency component of relatively low frequency, its upper harmonicfrequency components extending to several thousands of cycles. Thefrequency of this fundamental component is the rate at which the humanvocal cords vibrate, and in general, although it can be held constant bysustaining a continuous sound, it continuously increases or decreases asthe inflection of the voice rises or falls. This type of signal has beencharacterized as the voiced sound and includes the vowel and near-vowelsounds. In it the energy is distributed in discrete subgroups, or bands,within the frequency spectrum of the signal. These subgroups, or bands,are separated by a frequency interval that is numerically equal to thefrequency of the fundamental component of the signal, and the energycontent of the subgroups generally decreases as their frequency isincreased. In the second type of signal, there is a continuous andgenerally random distribution of the energy through the frequencyspectrum, as distinguished from discrete subbands in integral harmonicrelation. This second type of signal has been characterized as theunvoiced sound and may be considered to be the limiting case of thefirst pattern type as its fundamental frequency approaches zero cyclesper second. In addition, although the energy Yis distributed throughoutthe frequency spectrum of the signal, the unvoiced sound often containsthe major portion of its energy in the middle or upper half of itsfrequency range. In distinguishing between these two types of signalpatterns, advantage may be taken of the fact that in the voiced soundthere is a relatively high power level, while in the unvoiced sound thepowel level is usually much lower. In the so-called vocoder type oftransmission system, such as is disclosed in the above-mentioned Dudleypatent, this difference in power level of the signals is utilized todistinguish between the voiced and the unvoiced sound, In addition, therelatively high power level of the voiced soundpermits the production ofa frequency pattern signal to denote the instantaneous frequency of thefundamental component of the original speech signal. This frequencypattern signal, sometimes designated as the pitch control signal, istransmitted valong with the amplitude control pattern signal to thereceiving point to control the synthesizing apparatus of this system.For a more complete description of the operation of this type of system,reference may be had to the above-mentioned Dudley patent.

It has been suggested that this frequency pattern, or pitch controlsignal, be derived by beating, or heterodyning, adjacent frequencycomponents of the speech signal to produce a difference frequency thatis equivalent to the instantaneous frequency of the signals fundamentalcomponent. Such a system is capable of satisfactory operation where thefundamental component of the signal is actually present in the analyzedsound, of if not present, where the adjacent harmonic components are ofsufficient relative magnitude and constancy of phase relation to permita satisfactory heterodyning procedure. It is often desirable to transmitthe original speech signal a considerable distance from its originatingpoint before analyzing the signal to determine its frequency andamplitude pattern. In such cases, it may frequently happen that thefrequencies in the range of about 80 to about 250 cycles per second areconsiderably reduced by virtue of the attenuation characteristic of thetransmitting medium, or they may be subjected to considerable relativephase shift before their analysis. Under such circumstances thederivation of the frequency pattern signal may present considerabledifliculty, since the fundamental and possibly the lower ones of itsharmonic components have been effectively eliminated, and the relativephase and amplitude relations between the remaining adjacent harmoniccomponents may be shifting. In this latter case, it occasionally happensthat the adjacent harmonic components momentarily assume a relativeamplitude such that the heterodyning process derives a differencefrequency between non-adjacent harmonic components. In such a case, thederived frequency pattern signal indicates a shift of one or moreoctaves in the fundamental component, whereas in fact no change hasactually occurred. In View of the foregoing,.it appears desirable todetermine the frequency of the fundamental component of such a signalwave by a method and means that are not dependent upon the presence L inthe signal wave of the fundamental component, and which are not affectedby this occasional shifting of the phase and amplitude relations betweenthe various adjacent wave components.

v- It is a feature of the invention that it may be practiced through theuse of a relatively simple and inexpensive arrangement for scanning aportion of the signal wave to determine the fre- .quency intervalbetween observed harmonically related wave components in such a mannerthat the final result is not affected by relative phase and amplitudedisplacements of the components.

It is also a feature of the invention that the derivation of the pitchcontrol signal is not dependent upon the high energy ylevel of the lowfrequency signal component, and, consequently, the presence or absenceof the lower ones of the harmoniccomponents is not a necessary precedentfor the successful practice of the invention.

It is a further feature of the invention that the distinction betweenvoiced and unvoiced signals does not depend upon a difference in thepower level of the original signal. Other objects and features of theinvention will be apparent from the following description of a preferredembodiment, when considered in conjunction with the drawings, in which:

Fig. 1 is a block schematic diagram of 'an embodiment of the inventionwhen arranged for voperation in a frequency compression signaltransmission system;

Figs. 2, 3 and 4 are explanatory graphs to which references are made inthe description of the operation of thearrangement of Fig. 1;

. Fig.-5 is a schematic diagram of a second em- CFI bodiment of theinvention such as might be employed in a frequency compression signaltransmission system; and

Fig. 6 is an explanatory diagram indicating one difference in theoperating characteristics of the Fig. 1 and Fig. 5 embodiments of theinvention.

Fig. 1 shows an embodiment of the invention as being incorporated in afrequency compression type of signal analyzing and synthesizing systemsuch as was disclosed in the above-mentioned United States Patent2,151,091, March 21, 1939, to H. W. Dudley. In this gure, the delayequalizer DE and the amplitude pattern control equipment III function toderive indications of the amplitude of the signal energy in a number ofpredetermined subbands of the original signal wave. For a completedescription of the operation and structure of this branch of thecircuit, reference may be made to the above-mentioned Dudley patent. Thedetermination of the frequency, or pitch, of the fundamental componentof the signal wave is made in the upper branch circuit which comprisesamplier I4 connected to the signal source over path I2. Variableoscillator I8 supplies modulating energy to the balanced modillator I6,and band-pass filter 20 selectively attenuates a portion of themodulator output before its rectification in rectier 22. Limiter 24removes amplitude variations from the output of rectifier 22 before thehigher frequency components therein are attenuated in the low-passfilter 26, and before the direct current component is removed bytransformer 28. The frequency selective network comprising resistor 29and capacitor 30 is followed by an amplifier 32 and rectier 34. jLow-pass filter 36 eliminates substantially all alternating currentcomponents before an output voltage is produced across the load resistor38. Low frequency oscillator 40 may have its frequency of oscillation`determined by the magnitude ofthis outputvoltage such that the pitchcontrol, or frequency pattern, signal is in a convenient form fortransmission purposes.

In its operation, the signal analyzing arrangeinent of Fig. `1 receivessignal waves, which, for example, may be speech signals of the voicedtype, from the signal input at the left of the drawing. These signalsare simultaneously applied through the delay equalizer DE to theamplitude pattern control equipment II), and over connecting path I2 tothe input of amplifier I4 in the pitch determining branch. Amplifier I4serves to isolate this branch from the remainder of the system andinsures that the signal will be of a suitable level before itsapplication to one input circuit of the balanced modulator I6.Oscillator I8 provides a linearly changing frequency supply to the`second input circuit of modulator I6. As indicated in Fig. 2, forexample, the frequency of `oscillator I3 may be caused to vary in asaw-tooth fashion between frequencies of 400 and 900 cycles per secondduring each two-tenths second interval. If desired, these frequencies,or the time interval, or both may be changed to satisfy varyingoperating conditions. The combination, in modulator I6, of the inputsignal wave, comprising wave components f1, f2, fs. etc. in integralharmonic relation to a fundamentalcomponent lio, which may or may not bepresent, and the linearly changing modulating frequency from oscillatorI8, produces linearly changing sum and difference frequency outputpatterns of modulation in which the energy is grouped in discretesubbands having the same-frequency separation interval as did componentsf1,.f2, etc. of the orig- Yinal signal wave. In the normal voiced speechsignal, this frequency separation may vary from about 80 to about 250cycles per second, for the more important voiced speech signals. Thefilter 20 may be ofthe band-pass-type having a suitable pass-band suchas, for example, 50 cycles centered-at about 950 cycles per second.However constituted, the width of the filter 'pass-band should be lessthan the minimum frequency separation between energy subbands, which fornormal speech signals should be about 80 cycles per second. Filter 2nrejects one of the modulation products which, in this assumed example,would ,be the difference frequency, and selects from the changing sumproduct a band of frequencies about -50 cycles in width. As thefrequency output from oscillator I8 is changing from, for example 400 to900 cycles per second at the rate of 2500 cycles per second, themodulation product will also be changing at that rate, and subbands ofvenergy -will be selected from vthis changing ,modulation product byfilter 20 in a scanning manner. Since these subbands of energy arelocated fo cycles per second apart, the Alter 20 will select bursts, orpulses, of energy from this modulation lproduct at the rate of andvaries directly as the frequency of the fundamental component fo varies.Amplitude limiter 24 removes any amplitude differences from thesepulsations, and low-pass filter 26 rejects all wave components in excessof a suitable maximum frequency such as, for example, 500 .cycles persecond. Passage through transformer 23 removes the direct currentcomponent from this rectified output, and results in a purified wave 44of approximate sine character, as indicated in Fig. 4. This wave 44 isvariable in frequency and has the same period P between its adjacentpoints of maximum amplitude, at any instant, als did voltage pulsationsfrom the bridgerectifier 22. Assuming that' the original speech signalwave had a range of fundamental frequency from 80 to about 250 cyclesper second, this purified wave 44 after passage through transformer 28varies in inverse relation from about 31 cycles to 10 cycles persecond.. Resistor 29 and capacitor 30 form a frequency-sensitive networkhaving an inverse frequency-attenuation characteristic such that thehigher frequencies of the purified wave 44 are attenuated to a greaterextent than are the lower frequency components of this wave. Amplifier32 restores the level of the selectively attenuated derived wave beforethe second bridge rectifier and low-pass filter 36 convert it into aslowly varying unidirectional voltage appearing across the load resistor38, Low-pass lter 3 6 may have an upper frequency limit of aboutlOcycles per second, and removes substantially all of the alternatingcurrent components from the rectified signal. This provides a band widthsignal wave.

capable'of passing changes of inflection in the original signal, whichchanges occur at syllabic rates not usually in excess of eight times persecond. As indicated by curve 42 of Fig. 6, the amplitude of this outputvoltage is inversely related to the frequency of the purified lowfrequency wave 44, obtained at the secondary terminals of transformer28, and is directly proportional to the frequency of the fundamentalcomponent of the original input voiced signal wave. This slowly varyingoutput voltage may be used to control the frequency of oscillator 40 toderive therefrom a constant amplitude, variable low frequency signal,say, for example, 10 to 60 cycles per second, suitable for transmissionover the limited frequency line along with the currents derived from theamplitude pattern control equipment I0.

From the foregoing description it will be apparent that a `pitchdetermining arrangement in accordance with this invention is insensitiveto the unvoiced type of speech signal. It will be recalled that theunvoiced signal wave has its energy content randomly distributedthroughout the spectrum of the signal, with any concentration of energygenerally occurring in the middle or upper half of the spectrum. Whensuch a wave is modulated by the variable frequency modulating energyfrom oscillator I8, the modulation product retains this random energyldistribution, which distribution provides a very small, if any,separating interval between energy subbands. This condition may be saidto approximate the limiting condition of zero cycles per second for thefundamental of a voice When such a modulated output wave is scanned bythe scanning band-pass lter 20, and is rectified in rectifier 22, thereresults either a constant value of unidirectional voltage output, or anoutput of unidirectional voltage in which there is such a smallseparating interval between the maximum amplitudes that it appears tohave a very high frequency alternating current component. In eitherevent, the combined action of the low-pass filter 26 and transformer 28,acting to eliminate all components above 500 cycles per second and alldirect current components, reduces this rectified output to a negligiblefactor, and renders the circuit effectively insensitive to unvoicedsignal waves. Under some operating conditions it may be found that thelow frequency end of the derived purified wave 44, obtained after thehigher frequency components and the direct current components have beenremoved from the output of retier 22, is sufliciently low to impartundesired voltage fluctuations to .the voltage output of filter 33 andas derived across the load resistor 38. In Fig. 5 there is shown anembodiment of the invention which provides means for accommodating thefull frequency range of the input signal, and also provides increasedfrequency at the low frequency end of the purified low frequency wave.This arrangement provides increased margin between the lowest frequncyof the derived wave 44, and the cut-off frequency of filter 35 byincreasing the scanning rate of filter 20 as the frequency of' thefundamental component of the original signal wave increases.

The previously describedv arrangement of Fig. l.

the vonage output from ampuner 52. This' os" livio-iii In its method ofoperation, this Fig. 5 embodiment closely follows the previouslydescribed procedure of the Fig. 1 embodiment. Increased margin betweenthe lowest frequency of the derived purified wave and the upper cut-offof low-pass filter 36 is secured by tapping the output voltage appearingvacross load resistor 38 l at the upper end of the resistor. Thisvoltage may be supplied over path 50 to the directcoupled amplifier 52,the amplified output of which controls the operating frequency of re'-laxation oscillator 54. Oscillator 54 produces saw-tooth shaped outputvoltage waves, at a frequency that may be in direct proportion. to themagnitude of the amplified voltage from amplifier` 52. When impressedupon the oscillator control 56, the saw-tooth wave operates to cause thevariable'oscillator I8 to progressively change its frequency from itslower to upper frequency limit. Therefore, during any' particular timeinterval, .the number of frequency excursions by oscillator I8 will bedetermined by the amplitude of the output voltage appearing acrossresistor 38. Increasing the number of frequency excursions of oscillatorI8 effectively increases the number of cycles per second that arescanned by band-pass lter 20, and changes the separation between energysubbands as seen by this filter from the previously described relationai fu to a new relation of where X is a variable quantity in excess of2500 by some arbitrary quantity, the exact value of which depends uponthe frequency of the fundamental component of the applied signal wave.In this manner, the energy subbands, as seen by the scanning filter 20,change from a separation interval equal to l f., for the lowestfrequency fundamental component of say 80 cycles to a variableseparation interval of as the higher fundamental components increase infrequency. In this manner there is produced at the secondary terminalsof transformer 28, a derived purified wave the frequency of which isrelated to the instantaneous value of the fundamental wave component,but in which the minimum frequencies are of a higher value than in thepreviously described embodiment. As indicated by curve 58 of Fig. 6, theoutput voltage appearing across load resistor 38 has lower amplitudes atvalues corresponding to the higher fundamental frequencies of theapplied signal wave. Curves 42 and 58 (Fig. 6) indicate the comparativeeffectA upon the output voltage of increasing the scanning rate indirect relation to the frequency of the fundamental component of thevoiced input signal.

As in the case of the embodiment of Fig. 1, the amplitude of the outputvoltage at resistor 38 is directly related to the frequency of thefundamental component of the original signal wave. As previouslydescribed, this voltage may be used to control the frequency of lowfrequency oscillator 48, to produce a pitch-defining signal havingsuitable characteristics for transmission through or over the availablemedium as indicated in the above-mentioned Dudley patent.

Although the invention has been described in embodiments that wereespecially adapted for association with the ,so-called vocoder type. ofsignal analyzing and synthesizing system, it should be appreciated thatits utility `is 'not limited to this type of system.. lRather, it hasgeneral utility wherever a periodicy or quasiperiodic complex wave is tobe analyzed' to determine. the frequency of its fundamental component,regardless of whetherlthe'fundamental component and its lowerharmnically related component are actually present in the analyzed wave.

What is claimed is:

1'. A system for producing an indication of the frequency of themissin-g fundamental component of a complex signal wave which includes aplurality of harmonic components in integral rela-'- tion to'saidfundamental component, which comprises selective means for repetitivelyselecting from a portion of said wave each discrete harmonic componentin said portion during a known" interval, unidirectional conducting andtranslating means for deriving from said selected components a series ofunidirectional voltage impulses, means for deriving from said impulses asmoothly varying alternating voltage, and v'means for converting saidderived alternating voltage into a unidirectional voltage, the amplitudeof which is proportional to the frequency of said missing fundamentalcomponent.

2. Apparatus for producing an indication of the frequency of thefundamental component of a complex signal wave which includes aplurality of harmonic components in integral relation to saidfundamental component, which comprises a source of oscillations thefrequency of which is repetitively varied between designated limits at aknown rate, means for combining said signal wave with said Variablefrequency oscillations to produce at least one sideband product,thefrequency of which repetitively .linearly varies beg tween designatedlimits in a known interval,

selective means for segregating from a portion of said sideband eachdiscrete harmonic component in said portion during said interval,unidirectional conducting and translating means for converting saidsegregated components into a series of unidirectional voltage impulses,amplitude limiting and, frequencysensitive means for converting saidimpulses to an alternating voltage the frequency of which is indicativeof the number of harmonic components that were segregated in acorresponding interval, and unidirectional conducting means for derivingfrom said variable frequency alternating voltage a unidirectionalvoltage the magnitude of which is indicative of the frequency of thefundamental component of said wave, u

3. The method of producing an indication o the frequency of thefundamental component of a complex signal wave comprising a plurality ofwave components in integral harmonic frequency relation to saidfundamental component, which comprises repetitively scanning a portionof said wave to repetitively segregate each harmonically related wavecomponent, detecting said harmonically related wave components,producing a unidirectional voltage the magnitude of which isproportional to the intervals between adjacent successively detectedcomponents, and varying the rate at which said wave is scanned inaccordance with the changes in said interval of separation.

4. Apparatus for producing an indication of the frequency of thefundamental component of a complex signal wave having a plurality ofwave components in integral harmonic relation to said fundamentalcomponent, which comprisesra source of signal waves, a source ofoscillations the frequency of which repetitively varies betweendesignated frequency limits at a substantially linear rate during a timeinterval, modulatory means for combining said signal wave with saidvarying frequency oscillations to produce sideband 'products ofmodulation, means for selecting from a predetermined portion of saidmodulation products each harmonically related wave component within saidportion during said time interval, means responsive to the number ofsaid wave components that were selected during a given interval forproducing a unidirectional voltage the magnitude of which is indicativeof the frequency of the fundamental component of said signal wave, andmeans responsive to the amplitude of said frequency indicating voltagecluding means responsive to said unidirectional output voltage forchanging the rate of variation of said source of repetitively varyingoscillations in accordance with the magnitude of said output voltage.

7. The method of producing an indication of the frequency of thefundamental component of a complex wave including a plurality of wavecomponents in integral harmonic relation to said fundamental component,which method comprises the steps of (l) segregating each harmonic wavecomponent in a predetermined frequency portion of said wave once duringeach of successive time intervals, (2) producing from said segregatedcomponents a train of voltage impulses, each of said impulsescorresponding in time to the segregation of one of said components, and(3) producing from said train of impulses a fluctuating unidirectionalvoltage the magnitude of which is at any instant indicative of theinterval between successive ones of said impulses and of the frequencyof said fundamental component.

8. In a system for producing an indication of the frequency of thefundamental component of a complex signal wave including a plurality ofwave components in integral harmonic relation to said component, thecombination which includes a frequency selector, frequency translatingmeans for impressing on said selector the heterodyned product of saidcomplex wave and a wave whose frequency is linearly varied betweenpredetermined minimum and maximum values during recurring intervals ofknown duration, means for detecting the wave components that aresegregated by said frequency selector, means for controlling the rate atwhich said oscillatory source changes in frequency.

5. Apparatus for producing an indication of the frequency of thefundamental component of a complex signal wave having a plurality ofwave components in integral harmonic relation to said fundamentalcomponent, which comprises a source 0f signal waves, a source ofrepetitively varying oscillations, modulatory means for cornbining saidsignal waves and said oscillations, selective means for segregating aportion of said combined waves, unidirectional conducting means fordetecting said wave components in said selected portion, means forconverting said detected wave components into a relatively low frequencyalternating wave in which the frequency of alternation is related to therate at which said wave components were detected, frequency responsivemeans for attenuating said alternating wave in accordance with itsfrequency of alternation, and unidirectional conducting means forconverting said alternating wave into a unidirectional output voltagethe amplitude of which is indicative of the frequency of the fundamentalcomponent of said signal wave.

6. Apparatus in accordance with claim 5 infor converting said detectedcomponents to an alternating voltage the frequency of which isproportional tothe number of wave components that are segregated by saidselector in a unit interval, and frequency demodulating means forconverting said alternating voltage intoa unidirectional voltage themagnitude of which is indicative of the frequency of said fundamentalcomponent.

9. The combination described in claim 8, including means responsive tosaid derived unidirectional voltage for varying the interval duringwhich said lheterodyned wave is changed between its minimum and maximumfrequency values in accordance with changes in the magnitude of saidvoltage.

DOREN MITCHELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,976,481 Castner Oct. 9, 19341,994,232 Schuck, Jr Mar. 12, 1935 2,151,091 Dudley Mar. 21, 19392.465.355 Cook Mar. 29. 1949

